1. Field of the Invention
The present invention relates generally to the measurement of intracranial pressure. More specifically, this invention relates to the provision of an improved method and apparatus for the non-invasive measurement of changes in intracranial pressure.
2. Brief Description of the Prior Art
Measurement of intracranial pressure (ICP) has become a routine neurosurgical procedure used in monitoring patients with conditions such as head injury, intracranial infection, hemorrhage, and hydrocephalus.
Generally when ICP reaches 20 mm Hg it becomes a concern and when it reaches 25 mm Hg for more than 2 minutes it is considered life threatening. Normal ICP is considered to be 0-4 mm Hg.
There are a number of prior references on the measurement of fluid pressures within the body cavity. The applicable work known to applicant is set forth in the following references:
Murr, U.S. Pat. No. 3,853,117--fluid pressure within a body cavity is measured with the use of a sonic transponder implanted inside the body cavity, e.g., the cranium. A sound signal is sent to the transponder to emit a resonance signal which is received at an external detector. The method is based on the principle that the skull and the cranial fluid are good sound conductors and that there are no intervening structures to attenuate the input or output signals. The implanted transponder has a diaphragm which serves as a mechanically resonant structure. This technique has major drawbacks, however. It involves invasive methods which introduce significant risks due to the possibility of cerebral infections. Furthermore, the measurement of a sound signal by an external detector affects deleteriously the measured sound's signal to noise ratio due to background noise present in the environment and signal strength attenuation inherent in the use of external detectors to receive sound signals. Lastly, the characteristics of the measured sound (accoustic signal) signal are only indirectly related to changes in ICP and thus may give rise to inaccurate results.
Pratt, U.S. Pat. No. 4,361,154--describes a method of determining bone strength by measuring the relative speed of travel of sound through the bone. The method has particular application in determining the bone strength of a race horse's legs. The method is based on the principle that microcrushing and microfracturing which occur in bone over time in the process of absorbing shock results in a decrease in sound velocity measured across a section of the bone. Since the elastic modulus of bone is known to decrease as it weakens, there is a relationship between bone strength and rate of travel of acoustic energy through the bone. This patent, however, does not appreciate the measurement of ICP.
Rosenfeld, et al., U.S. Pat. No. 4,564,022--involves a method of non-invasively estimating ICP whereby electrical brain activity is generated by a stimulus. In particular, the patient's observance of a flashing light results in visual stimulation which causes visual evoked potentials signalled from the brain. These visual evoked potentials are measurable and their characteristics are well-defined in both children and adults. An accurate estimate of the subject's ICP can be made by measuring the latency of the second negative-going wave of the visual evoked potential. The method used to arrive at this estimate, however, is based upon the measurement of characteristics which are secondarily related to ICP. This may give results which are less accurate than those which may be obtained by measuring characteristics which are primarily related to ICP, such as vibration frequency characteristics.
Cosman, U.S. Pat. No. 4,676,255--this disclosure is a continuation-in-part of a patent relating to measuring ICP by using known pressure applied to the scalp over an implanted sensor. This particular patent focuses on measurement of negative ICP. Unfortunately, the problems which are inherent in the use of implanted (invasive) sensors are also present in this system.
Sackner, U.S. Pat. No. 4,860,766--provides a non-invasive method of monitoring intrapleural pressure of a newborn. The method is based on the principle that the cranial bones of a newborn move relative to each other during respiration as a result of a pressure wave transmitted from the pleural space through the cerebrospinal fluid and veins to the cranial cavity. The movements of the cranial bones are detected and monitored and the waveform produced directly relates to intrapleural pressure. This patent, however, does not offer any teaching that would allow one skilled in the art to measure ICP.
U.S. Pat. No. 1058-556-A--this Soviet patent describes a non-invasive method of measuring ICP whereby an ultrasound sensor is positioned on one side of the front of the cranium and a pulsing signal is sent through the cranium to the occipital wall of the skull and back. The reflected ultrasound signal is recorded and ICP is determined thereby based on a set of formulas relating to amplitude of echopulsation within the cranium. This method uses sound and, therefore, the problems present in the measurement of ICP using sound (for example, a poor signal to noise ratio) are also inherent in this method.
Devine, III, et al., IBM Technical Disclosure Bulletin--describes the measurement of internal pressure in a closed chamber within a living body whereby externally applied mechanical vibration is used to induce a differential Doppler by which internal pressure can be determined. The method is geared to the measurement of ventricular pressure in the heart. The frequency, amplitude and phase of the induced vibration are known, and the reflected ultrasonic energy is detected by a receiver. This technique uses sound detection and therefore in no way overcomes the difficulties, detailed previously, associated with the use of sound to measure pressure.
Kasuga, et al., "Transmission Characteristics of Pulse Waves in the Intracranial Cavity of Dogs," Journal of Neurosurgery, Vol. 66, Jun. 1987, pp. 907-914--discusses an attempt to mathematically model the intracranial pressure pulse wave transmission transfer function using the common carotid artery (randomized by using a cardiac pacemaker) as an input signal and the epidural pressure pulse wave as the output signal. The transfer function was estimated numerically from the autocorrelation function of the input signal and the cross-correlation function of the input and output signals by the least squares method. The results suggested that the lower frequencies of the pulse wave were suppressed during transmission through the intracranial cavity and that resonance was evident in the intracranial cavity under normal conditions. ICP could then be calculated using the transfer function and a known input signal. The technique used, however, has significant drawbacks. The greatest drawback is that the technique is invasive and thus involves the risk of intracranial infection. Furthermore, the technique is difficult to use, requires surgery, involves the dangers inherent in use of cardiac pacemakers, and presently is difficult to use in a clinical environment.
Kosteljanetz, et al., "Clinical Evaluation of a Simple Epidural Pressure Sensor," Acta Neurochirurgica, Vol. 83, 1986, pp. 108-111--This reference discusses the evaluation of Plastimed.RTM. epidural pressure (EPD) sensor in a number of patients suffering from head injury. The EPD sensor, a plastic cup 10 mm in diameter, was placed into a burrhole and two plastic tubes, one longer than the other, were connected to the cup. A pressure transducer was connected to the longer of the two tubes and the shorter tube was connected to saline reservoir via a stopcock. A conventional intraventricular pressure sensor (IVP) was situated next to the EPD sensor cup and recorded pressure continuously. The ICP values obtained from the IVP sensor were compared to those obtained from the EPD sensor. The comparison indicated that the EPD sensor gave inaccurate ICP readings. In addition to this drawback, the EPD system is invasive and requires careful alignment of the cup to the burrhole in order to obtain reasonable results. Furthermore, this method is subject to sudden sensor failure.
Takizawa, et al., "Spectral Analysis of the CSF Pulse Wave at Different Locations in the Craniospinal Axis," Journal of Neurology, Neurosurgery, and Psychiatry, Vol. 49, 1986, pp. 1135-1141--discusses a study intended to determine the change in frequency spectrum of the cerebrospinal fluid (CSF) pulse waveform, the amplitude transfer function from blood pressure to the CSF pulse and the conduction of each component of the CSF pulse through the CSF space under normal and abnormal conditions produced by saline infusion into the CSF space (thus elevating the pressure within the CSF space). Pressure transducers were positioned halfway between the tip of the dorsal spine and the sternum. CSF pulse and blood pressure under normal and artificially high CSF pressures were recorded at various sites. Several drawbacks are present in this method of pressure measurement. The method is invasive. Furthermore, the use of blood pressure as the input signal, with its discontinuous pressure values in between heart beats, can result in errors in the calculation of fluid pressure, since the transfer function obtained will be discrete rather than continuous.
Semmlow and Fisher, "A Noninvasive Approach To Intracranial Pressure Monitoring," Journal of Clinical Engineering, Vol. 7, March 1982, pp. 73-78--This reference discusses measurement of ICP by measuring the acoustical transmission properties of a skull under pressure. An impulse-like stimulus was applied to skulls suffering from various levels of elevated ICP. A piezoelectric acoustic pickup was used to monitor the acoustic response, resulting from the impulse stimulus, transmitted through the skull. Based on this monitored acoustic response, a second-order system response was modelled. Damping factor, a characteristic of a second-order system, was found to be indicative of elevated ICP. This method of ICP measurement, however, is subject to several problems. Acoustic transmission measurements, as previously mentioned, have poor signal to noise ratios. Furthermore, ICP measurements are derived, using this method, not from direct measurements but rather from artificial and often times inaccurate mathematical system model characteristics. This may lead to largely inaccurate ICP measurements.
Accordingly, the present invention generally has as its objective the provision of both a method and an apparatus for the measurement of ICP which overcomes the above-mentioned problems in the prior art. More specifically, the present invention has as an objective the provision of a method and an apparatus for the measurement of ICP which eliminates the risks of cerebral infections.
It is a further objective of the present invention to provide a method and an apparatus for the measurement of ICP which is not subject to large measurement errors due to poor signal to noise ratios of the measured signal.
It is a further objective of the present invention to provide a method and an apparatus that measure characteristics primarily related to ICP.